1. Field of the Invention
The invention relates to liquid crystal displays particularly with respect to controlling an integral heater thereof.
2. Description of the Prior Art
Liquid crystal displays (LCD) have found wide-spread usage in the prior art, one such display being, for example, of the twisted nematic liquid crystal type. Such LCDs operate by applying an alternating potential between the front and rear electrodes of the display. The RMS voltage of the applied potential determines if a display segment is on or off. The alternating potential must have an average DC level that is below the DC potential that causes electroplating between the front and rear electrodes. The electroplating process causes the LCD to fail due to destruction of the alignment coatings. It is appreciated, however, that significant electroplating does not occur at low temperatures but at high temperatures, electroplating can occur to cause dislay failure. At high temperatures the average DC voltage between the front and rear electrodes of the display must be maintained below the level that results in electroplating action. Present day integrated circuit power supplies for LCDs typically provide an alternating driving potential with such a low DC level that even at high temperatures the rate of electroplating is so minimal as to be undetectable.
Present day LCDs become inoperative at very low temperatures. At minus 10 to 20 degrees centigrade, the fluid in the LCD becomes so stiff that the LCD display elements do not turn on. Separate heaters have been provided with LCDs so that the device operates at low temperatures. Typically, one side of the heater is permanently connected to a DC potential and the other side of the heater is switchably connected to ground to activate and deactivate the heater in response to LCD temperature. The separate heater has a number of disadvantages. In order to heat the display fluid to a fixed temperature, the heater must be hotter than the desired display temperature. The difference in temperature between the display and the heater is proportional to the low temperature turn on time of the LCD. The hotter the heater, the faster the display will commence operation during a cold temperature startup. Since it is the fluid in the LCD and not the remainder of the unit that must be heated, most of the energy provided by the heater is wasted. Additionally, the time required to heat the fluid is proportional to the distance between the fluid and the heating element. The closer the heater is to the fluid, the faster the fluid can be heated. Thus, LCDs with separate heaters tend to exhibit sluggish cold temperature response.
Recently, commercially procurable LCDs have been manufactured with an integral heater element. This heater traditionally comprises a thin sheet of, for example, indium tin oxide (ITO) plated on the back of the rear glass plate of the display on the front of which are deposited the rear LCD electrodes. The heater element is typically sandwiched between the back surface of the rear glass plate and the rear polarizer of the device. By this arrangement the heat transfer characteristics are improved with respect to the separate heater configuration thereby providing fast cold temperature operation.
As previously described, traditional heater operation provides maintaining a DC voltage connected to the high side of the heater and switchably connecting the low side of the heater to ground. Alternatively, a ground connection can be permanently effected to the low side of the heater with the heater DC potential switchably applied to the high side. With this integral heater arrangement, a capacitor is formed with the front and rear LCD electrodes providing one capacitor electrode and the heater providing the other capacitor electrode with the rear glass that separates the front and rear LCD electrodes from the heater providing the capacitor dielectric. With an alternating potential applied between the front and rear LCD electrodes and the heater connected to a fixed potential, an average DC current flows between the heater and the LCD electrodes because of the current path established between the LCD electrodes and heater power or ground resulting from the fixed potential connection to the heater. At high temperatures this DC current flow results in sufficient electroplating action so as to damage the alignment coatings resulting in display failure. Because the failure mode is caused by electroplating, at low temperatures, even with the heater activated, the rate of electroplating is undetectible and it is appreciated that the heater is only activated at low temperatures. Thus, at high temperatures, the DC current flowing, even with the heater deactivated, is sufficient to result in a rapid degradation of the alignment coatings which causes premature LCD failure. The rate at which the electroplating occurs that results in failure is proportional to the temperature of the crystal and the average difference in DC potential between the heater and the front and rear LCD electrodes. The higher the DC potential and the higher the temperature, the greater is the rate of electroplating and hence the greater is the rate of display failure.
The phenomenon that results in failure can also be appreciated by considering that the rear LCD electrodes form a capacitor with the heater, the front LCD electrodes form a capacitor with the heater, and the front and rear LCD electrodes form a capacitor with respect to each other with the LCD fluid acting as the dielectric therebetween. The current path provided by the fixed potential connected to the heater is principly through the rear LCD electrodes and therefore the alternating currents across the rear electrode capacitor and the front electrode capacitor flowing through the fixed potential connection are different. This creates an imbalance with respect to the capacitance between the front and the rear LCD electrodes resulting in a net DC current flow.